1. Field of the Invention
The present invention relates to an ophthalmological apparatus and an alignment method.
2. Description of the Related Art
An eye examination is widely performed for the purpose of early diagnosis of lifestyle-related diseases and diseases ranked high as causes of blindness. Scanning laser ophthalmoscopes (SLOs), which are ophthalmological apparatuses based on the principle of confocal laser scanning microscopy, are apparatuses that perform a raster scan on the fundus with a laser beam serving as measurement light and capture a high-resolution plane image at a high speed on the basis of the intensity of the returning light.
Hereinafter, an apparatus that captures such a plane image is referred to as an SLO apparatus and the plane image is referred to as an SLO image.
Recently, it has become possible to capture SLO images of the retina with an improved lateral resolution by increasing the diameter of a measurement light beam in SLO apparatuses. However, the increased diameter of the measurement light beam has caused, when SLO images of the retina are captured, an issue of decreased signal-to-noise (S/N) ratio and resolution of the SLO images due to the aberrations of the subject's eye.
To address this issue, adaptive optics SLO apparatuses have been developed. Adaptive optics SLO apparatuses include an adaptive optics system configured to measure, with a wavefront sensor, the aberrations of a subject's eye in real time and to correct, with a wavefront correction device, the aberrations of measurement light and its returning light caused by the subject's eye. Such adoptive optics SLO apparatuses can capture SLO images with a high lateral resolution.
In order to capture such high-lateral-resolution SLO images as a moving image and noninvasively observe blood flow, for example, retinal blood vessels are extracted from each frame and the speed at which blood cells move in capillaries or the like is measured. Also, in order to evaluate the relationship between the visual performance and a density distribution or array of photoreceptor cells P using SLO images, the photoreceptor cells P are detected and the density distribution or array of the photoreceptor cells P is measured. FIG. 6B illustrates an example of a high-lateral-resolution SLO image. The photoreceptor cells P, a low-luminance region Q which represents positions of capillaries, and a high-luminance region W which represents a position of a white blood cell are observable.
When the photoreceptor cells P are observed or the distribution of the photoreceptor cells P is measured in the SLO image, an in-focus position is set to be in the vicinity of the retinal outer layer (B5 in FIG. 6A). In this state, an SLO image such as the one illustrated in FIG. 6B is captured. Along the retinal inner layers (B2 to B4 in FIG. 6A), retinal blood vessels and capillaries branching from the retinal blood vessels run.
As an alignment technique applied to a low-magnification image Dl and a high-magnification image Dh of the eye, a technique for displaying a capture position of an adaptive optics SLO image on a low-magnification image of the fundus is disclosed in Japanese Patent Laid-Open No. 2010-259543.
When a high-lateral-resolution image (high-magnification image Dh) is superimposed on a wide-angle image (low-magnification image Dl) (see FIG. 6C), alignment is sometimes not accurately achieved because these images lack a common image feature owing to their greatly different angles of view and pixel sizes.
Accordingly, a technique is desired which allows for accurate alignment of images that have greatly different angles of view and pixel sizes by capturing an intermediate-magnification image Dm including an image feature in common with the high-magnification image Dh and an image feature in common with the low-magnification image Dl and by performing alignment using the intermediate-magnification image Dm.
Also, as illustrated in FIG. 6D, a capture position or capture range of a tomographic image is sometimes superimposed on the low-magnification image Dl of the eye. The resulting image is typically displayed along with the tomographic image (FIG. 6A) and is used to observe a layer shape of the eye while checking the capture position of the tomographic image.
If the high-magnification image Dh is accurately aligned on the low-magnification image Dl, the capture position of the tomographic image can be superimposed on the high-magnification image Dh. This thus makes it possible to observe and analyze a relationship between a distribution of the photoreceptor cells P in the high-magnification image Dh (for example, a range of defective photoreceptor cells) and the layer shape of the eye (for example, thinning of the retinal outer layer).
In addition to the capture position of the tomographic image, i) a distribution of layer shape values (FIG. 6E), ii) a distribution of retina sensitivity values (FIG. 6F), and iii) a position irradiated with a therapeutic laser beam are sometimes superimposed on the low-magnification image Dl of the eye. If the high-magnification image Dh is accurately aligned on the low-magnification image Dl, a relationship between an image feature in the high-magnification image Dh and these pieces of information can be observed and analyzed. For example, when the information i) is used, a relationship between a distribution of the retinal outer layer thickness and a density distribution of photoreceptor cells can be observed and analyzed. When the information ii) is used, a relationship between a distribution of visual performance measurement values and a density distribution of photoreceptor cells can be observed and analyzed. When the information iii) is used, a relationship between a treatment-target site and a distribution of capillaries or a blood flow speed can be observed and analyzed.
The technique disclosed in Japanese Patent Laid-Open No. 2010-259543 uses a tracking technique to associate relative positions of the low-magnification image Dl and the high-magnification image Dh with each other. However, Japanese Patent Laid-Open No. 2010-259543 does not disclose any image-processing-based alignment technique which also addresses the case of subjects with unstable fixation.
Japanese Patent Laid-Open No. 2009-276327 discloses a technique for alignment and display of a cell image captured with a full-field optical coherence tomography (OCT) and a low-magnification image of the fundus. However, Japanese Patent Laid-Open No. 2009-276327 does not disclose any image-processing-based alignment technique which also addresses the case where images having greatly different pixel sizes lack a common image feature and the case of subjects with unstable fixation.
Japanese Patent Laid-Open No. 2007-117714 discloses a technique for displaying a capture position of a tomographic image on an image of the fundus. However, Japanese Patent Laid-Open No. 2007-117714 does not disclose any technique for displaying a capture position of a tomographic image or a retina sensitivity distribution on the high-resolution image Dh of cells.